This invention relates generally to ammunition for expendable breech guns and more particularly to a round having optimum interior ballistics.
The present invention is designed to be used in expendable breech guns which employ the fire-out-of-battery (FOOB) recoil cycle such as that shown in the copending application of W. M. Moscrip, Ser. No. 606,906, filed Aug. 20, 1975.
Since the operation of the FOOB recoil cycle is in a dynamic sense "tuned" to the magnitude of the expected firing impulse, a high degree of reproducibility in the firing impulse from round to round is essential. The standard deviation of muzzle velocity (a measure of firing impulse reproducibility) in conventional guns is on the order of 1 or 2 percent under the best conditions. This variability is due to the normal variation of a large number of contributing factors such as propellant quality, initial grain orientation, surface area regression, projectile travel, etc.; it significantly degrades the total gun system accuracy and inherently complicates the design of a FOOB recoil system.
For low pressure guns maximum firing impulse reproducibility can be achieved by interposing an orifice plate between the propellant charge and the projectile. This is the high/low interior ballistics principle in which the burning of the propellant within the high pressure chamber proceeds independently of conditions in the bore, since the flow through the orifices is choked. Variations in muzzle velocity are reduced since the effects of projectile travel are isolated from the propellant burning environment and the gas production rate or flow rate is metered by the choked flow condition. However, the high/low principle is applicable only to low pressure systems and necessarily involves the use of a costly and relatively heavy high pressure chamber.
A measure of the interior ballistic performance of a gun system is provided by the ratio of the mean base pressure required to achieve a given projectile velocity to the actual maximum space mean pressure for the system. This parameter is known as the piezometric efficiency of the ballistic cycle and reflects how well the cycle maintains the maximum operating pressure during the entire action time. It has been theoretically demonstrated that optimum piezometric efficiency can be achieved in the operation of a closed breech gun only when the burning surface area development of the propellant grains is directly proportional to the projectile velocity.
A new hypervelocity research tool for achieving projectile velocities in excess of 35,000 fps and hopefully as high as 100,000 fps has been developed. It utilizes a hypervelocity accelerator consisting of a long thick-wall gun tube lined with a thin layer or film of explosive. The explosive lining serves as a reservoir of high pressure gas that is released by the passage of the projectile. The gas forms an essentially stationary reservoir that maintains a relatively constant base pressure on the projectile. In this application, very high projectile velocities have been achieved by using lightweight projectiles in long explosive lined tubes and by drawing a vacuum ahead of the projectile prior to each shot.